The present invention relates to testing equipment for telecommunications equipment. More particularly, and not by way of any limitation, the present invention is directed to an AC power fault machine capable of testing telecommunications (telecom) line cards and broadband coaxial cable interfaces to known power fault immunity criteria.
Telecommunications (telecom) equipment deployed in today""s networks is required to comply with various governmental and industry standards not only to ensure seamless interoperability which reduces the risk of service interruption resulting from third-party product failures, but also to address various product safety issues. Accordingly, equipment manufacturers test their products to telecom industry standards commonly known as Telcordia specifications (formerly known as the BellCore specifications) which define an extensive list of electromagnetic compliance (EMC), product safety, and environmental requirements.
The Telcordia specifications comprise two sets of testing standards, GR-1089-CORE and GR-63-CORE. The tests in GR-1089-CORE deal primarily with electrical phenomena, whereas the tests in GR-63-CORE are predominantly environmental in nature. While each set of standards is quite extensive, typically only a subset of the tests are required based on the type of equipment and its intended operating environment. Together, these two sets of standards specify the electrical and environmental requirements that networking hardware must meet in order to be located in a telco building, e.g., the telco""s central office (CO).
Besides the testing requirements, which are determined by product type, Telcordia has defined additional testing levels generally referred to in the telecom industry as Network Equipment Building Systems (NEBS) levels. The appropriate NEBS level for particular equipment is determined, again, by its intended operating environment and specific requirements of the Regional Bell Operating Companies (RBOCs). Generally, a higher NEBS level indicates a more stringent testing specification. NEBS testing verifies that telecom equipment can operate successfully under certain electrical and physical environmental stresses and not pose a safety hazard to personnel and users. These stresses and hazards include earthquakes, airborne contaminants, fire and smoke, electromagnetic interference (EMI), electrical safety, and grounding.
Requirements under the three NEBS levels may be summarized as follows: Level 1 includes: electrical safety; lighting and AC power fault (2nd level); bonding and grounding; emissions; and fire resistance; Level 2 includes: all of Level 1 in addition toxe2x80x94Electrostatic Discharge (ESD) under normal operation; emissions and immunity; lighting and AC power fault (1st level); ambient temperature and humidity (operating); earthquake Zone 2 and office vibration; and airborne contaminants (indoor level); Level 3 includes: all of Level 1 and Level 2 in addition toxe2x80x94ESD (installation and repair); open door emissions and immunity; ambient temperature and humidity (short-term); earthquake Zone 4; airborne contaminants (outdoor level); and transportation and handling. Each test within these three Levels is defined in either the GR-1089-CORE or GR-63-CORE documentation.
Testing of telecom interfaces, i.e., tip-and-ring (T and R or T/R) interfaces of the line cards utilized in telecom equipment and broadband coaxial cable interfaces, for lightning and AC power fault immunity in accordance with the above-referenced standards is necessary for several reasons. Power companies, Local Exchange Carriers (LECs) and broadband access providers often serve the same customers, and frequently employ joint-use facilities such as supporting structures or a common trench for their respective outside plant. Metallic conductors, such as cable or wire pairs serving telecom equipment may be exposed to electrical surges resulting from lightning and commercial power system disturbances. Despite the presence of protective devices in the telecommunications network that limit the effect of lightning and power surges, a portion of these disturbances can be impressed on the network equipment. Accordingly, under abnormal conditions, for instance, the power and telecommunications lines (including coax cables) may come into electrical contact. If the contact occurs to a primary power line, faults may be cleared quickly by the power system (5 seconds or less), and protectors (e.g., carbon blocks) can limit 60 Hz voltages appearing on the T and R conductors to maximum of approximately 600 VRMS with respect to ground. If the contact occurs to a secondary power line, the full secondary voltage with respect to ground (up to about 275 VRMS in some cases) may appear on the T and R conductors, which may persist indefinitely as the secondary fault may not be cleared by the power system.
Moreover, because electric power lines and telecom lines often occupy parallel routes as a result of a common right-of-way, the magnetic field produced by currents in a nearby power line, especially under abnormal conditions such as a phase-to-ground fault, may result in large voltages being induced into the telecom lines through electromagnetic coupling. The induced voltages appear longitudinally in the T and R conductors and may approach several hundred volts. Lower levels of induction may result from a high-impedance power fault such as a phase conductor falling to the earth. If the resulting unbalanced current is within the normal operating range of the power system, or if power system breakers or fuses do not operate, the fault may persist for an extended period of time.
Under the Telcordia""s GR-1089-CORE standard, the lightning surge and AC power fault immunity criteria include compliance with various tests such as short-circuit tests (tip to ring, tip to ground with ring open-circuited, ring to ground with tip open-circuited, tip and ring to ground simultaneously, et cetera) and several AC power fault tests. As set forth hereinabove, these criteria are separated into 1st level and 2nd level criteria. To comply with the 1st level criteria, it is required that the telecom Equipment Under Test (i.e., EUT) be undamaged and continue to operate properly after power stress is removed. To comply with the 2nd level criteria, primary protectors are typically removed and high open-circuit voltages and high short-circuit currents are often applied for variable durations, ranging up to 15 to 30 minutes or so in some instances. The EUT may sustain damage, but it is required that the equipment not become a fire, fragmentation (that is, forceful ejection of fragments), or an electrical safety hazard.
While several lightning machines are available for conducting the lightning compliance tests required under the BellCore standards alluded to hereinabove, there is a paucity of appropriate AC power fault (PF) machines capable of sourcing power to telecom units under test for adequately conducting the AC power fault compliance tests, including the 2nd Level tests. Further, the relatively few solutions extant today are beset with various shortcomings and drawbacks. First, the existing AC power fault machines are typically custom-designed to a large extent and, accordingly, incapable of accommodating various telecom equipment types and form factors. Additionally, these machines are quite expensive to manufacture owing at least in part to their custom design. In spite of the custom design, however, the existing PF machines are not capable of providing appropriate levels of test power safely to the EUT to conduct the whole range of 2nd Level power failure tests as required under the relevant Sections of the GR-1089-CORE standard. Furthermore, although the conventional PF machines are fairly capacious because of the large size of the transformers typically required to provide adequate levels of test power, they are incapable of sourcing power to both two-wire T/R interfaces as well as broadband coax cable interfaces in the same physical plant. In addition, it would be desirable to support testing the EUT interfaces while the EUT is operating under power.
Accordingly, the present invention advantageously provides a safe, versatile and single-platform power fault (PF) testing apparatus that is capable of performing both voltage mode and current mode testing on line card T/R interfaces (two-wire interfaces) as well as broadband coaxial cable interfaces to Telcordia""s 2nd Level AC power fault standards. The power fault testing apparatus for testing telecommunications equipment interfaces includes a variable autotransformer unit coupled to a three-phase power source through a power relay. The variable autotransformer unit operates to provide a selectably switchable power output. A fixed transformer is selectably coupled to the power output of the variable autotransformer unit in order to provide selectably switchable power output to an interface of an Equipment Under Test (EUT) that is operable to be disposed in a test chamber. The power output of the fixed transformer is referenced to a single point ground, which may be building ground. The single point ground may be positioned at a relay bank. The power fault testing apparatus of the present invention further comprises a return path for the power output of the fixed transformer from the EUT to the single point ground of the fixed transformer. The return path, thereby referencing the EUT to the building ground, allows for testing the EUT interfaces while equipment is operating under power.
In one embodiment, the power fault testing apparatus of the present invention comprises a load resistor bank disposed between the fixed transformer and the EUT""s tip-and-ring interface. The load resistor bank comprises a plurality of binary-coded resistors selectable by a series of relays coupled thereto. At least one relay operating under computer-based timer relay control may be disposed between the fixed transformer and the load resistor bank. Another relay operating under interface electronics control may be disposed between the variable autotransformer and the fixed transformer. A step transformer may be positioned between the variable autotransformer and the fixed transformer. The step transformer may be a 4:1 step down transformer. The variable autotransformer may be a 1:1.17 voltage boost transformer. The fixed transformer may be an isolation transformer.
In another embodiment of the present invention, at least one load resistor bank comprising individual (fixed) power resistors is disposed between the fixed transformer and the EUT""s coaxial cable interface. A boost transformer is disposed between the variable transformer and the fixed transformer. At least one relay operating under interface electronics control is disposed between the variable autotransformer and the boost transformer.
Alternatively, a step down transformer is disposed between the variable transformer and the fixed transformer in a voltage bucking configuration. At least one relay operating under interface electronics control is disposed between the variable autotransformer and the step down transformer. In another embodiment, the output leads of the fixed transformer are applied directly to the test chamber.
In another aspect of the present invention, the power fault apparatus for testing telecommunications equipment interfaces includes a three phase 480 Vac, 600 A power service for sourcing power to the power fault testing apparatus. A plurality of transformers and load resistor banks are selectively and switchably coupled to the power service in a network organized as a plurality of power paths for interfacing to one of a tip-and-ring interface and a broadband coaxial cable interface of an equipment under test (EUT). A portion of the plurality of transformers are referenced to a single point ground that is also referenced to the EUT. A plurality of power relays interconnect the transformers and load resistor banks disposed in the network. A plurality of output relays are positioned for effectuating duration control output power applied to the tip-and-ring and broadband coaxial cable interfaces. The relays are referenced to a ground bar positioned at a Remote Control Station (RCS) to prevent voltage drops, through a common return, from adversely affecting unpowered relays.
Preferably, the EUT is placed in a testing chamber comprising a clear Plexiglas enclosure. Additionally, preferably, the testing chamber is provided with a safety interlocking mechanism. In a presently preferred embodiment, a RCS is associated with the test chamber.